Ambient Cured Fly Ash Geopolymer Coatings for Concrete
Abstract
:1. Introduction
2. Theory
2.1. Geopolymer Fabrication
- Low cost: as fly ash is a by-product of coal combustion, it is cheap and available in large volumes around the world [30]. While there is a looming shortage of fly ash for use as a concrete additive [52], the availability for geopolymer coatings (a low volume application) is still high, as one billion tons of fly ash are still produced annually worldwide in coal-fired steam power plants [53].
- The mass ratio, L/A;
- The mass ratio, SH/SS;
- The molarity of the SH (which typically ranges from 8–14 M).
2.2. Factors Affecting Geopolymer Coatings on Concrete Substrates
2.2.1. Shrinkage and Curing
2.2.2. Adhesion, Workability and Setting Time
- High surface tension: this plays a key role in the ability of the materials to bind and stay bound to substrates. A reduction in surface tension may, in some cases, be required to ensure good adhesion [67]. With respect to geopolymer microstructure formation, the water to solid ratio can significantly affect the process of geopolymerization, and hence the properties of coatings, such as their workability and adhesion [5].
- Thixotropic Bingham plastic fluid behavior: most geopolymers show history-dependent rheological behavior, and can be kept in fluid form if subjected to constant shearing [13,68,69]. Their rheological behavior can also be tuned by altering the molar concentration of the sodium hydroxide and the ratio of silicate to hydroxide solutions [70].
- Setting times that are strongly dependent on chemical composition: the setting time of geopolymers can range from minutes to hours and depends on geopolymer composition. Setting times can be reduced by lowering Si/Al ratios, or by increasing calcium (Ca) content [61]. For systems with high Si/Al ratios, polymerisation is more likely to occur among silicate species; however, when Si/Al ratios are lowered, polymerisation is more likely to occur between aluminate and silicate species. As condensation among silicate species is slower than that between aluminate and silicate species, setting is delayed with higher Si/Al ratios [71].
2.2.3. The Concrete Substrate
2.2.4. Efflorescence
3. Materials and Methods
3.1. Materials
3.2. Methodology
3.2.1. Geopolymer Synthesis
3.2.2. Application to Substrate
- Newly cast, or “young” concrete samples, left to cure for 1–5 months;
- Intermediate-aged concrete samples, 5–12 months of curing;
- Old concrete samples, over 1 year of curing.
3.2.3. Concrete Substrate Roughness
3.2.4. Curing Conditions for Geopolymers
3.2.5. Analysis Methods
3.2.5.1. Vicat Needle Test
3.2.5.2. Isothermal Calorimetry
3.2.5.3. X-Ray Diffraction Analysis
3.2.5.4. Compressive Strength
3.2.5.5. Visual Inspection and Quantification of Cracks
4. Results and Discussion
4.1. Compressive Strength
4.2. Coating Thickness
4.3. Setting and Mixing Time
4.4. Concrete Age
4.5. Efflorescence
5. Further Discussion and Future Work
- Coating thickness: coatings with thickness < 1 mm showed no cracks, regardless of mixing times and RH levels between 50% and 95%. Coatings with higher thickness showed cracks, the extent of which was dependent on the mixing time;
- Mixing time: for our mix, the optimal mixing time was 1 h, to allow the main extent of geopolymerisation reactions to occur without loss of water or thermal stress, and to ensure that only few fly ash particles were unreacted. Optimizing mixing time can allow geopolymer coatings to overcome water loss induced cracking and to show a homogeneous surface without voids and bubbles.
- Age of concrete: the results showed that coating integrity did not depend on concrete age. This is an important consideration if geopolymer coatings and linings are to be applied to newly cast concrete structures, and not only to old structures which need repair.
- Efflorescence: for low-temperature, but humid curing conditions (above 70% RH), efflorescence was likely, as excess alkaline solution crystallized on the surface of the coating. Efflorescence was less likely for relative humidities at or below 50%.
5.1. The Role of Coating Thickness
5.2. Thermal Expansion and Bond Strength
5.3. Characterisation of the True SiO2/Al2O3 Ratio
6. Conclusions
Author Contributions
Funding
Conflicts of Interest
References
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Source | West Burton Power Station, Lincolnshire, England (UK) |
---|---|
Supplier | CEMEX |
SiO2 | 52.70 |
Al2O3 | 21.70 |
Fe2O3 | 7.10 |
CaO | 4.10 |
Loss on ignition | 4.20 |
Na2O | 1.10 |
K2O | 2.50 |
SO3 | 0.90 |
MgO | 1.80 |
Total phosphate | 0.58 |
Free CaO | 0.10 |
Median particle size, μm | 10.6 |
Age of Concrete | Young | Intermediate | Old |
---|---|---|---|
Surface roughness (mm) | 0.097 | 0.053 | 0.091 |
Batch | Curing Conditions | Temperature °C | Curing Time (days) | Average RH % |
---|---|---|---|---|
1 | Laboratory bench | 20 ± 2 | 28 | 50 |
2 | Environmental chamber | 20 ± 1 | 28 | 95 |
Phase Content | Mullite [%] | Quartz [%] | Magnetite [%] | Hematite [%] | Amorphous content [%] | Gypsum [%] | Rwp | |
---|---|---|---|---|---|---|---|---|
Sample | ||||||||
Fly ash (a) | 13.74 | 2.30 | 1.24 | 0.67 | 80.02 | 1.97 | 3.9 | |
Geopolymer (a) | 9.18 | 2.09 | 0.89 | 0.39 | 86.15 | 1.30 | 3.0 | |
Fly ash (b) | 15.68 | 3.48 | 1.25 | 1.34 | 76.65 | 1.36 | 2.4 | |
Geopolymer (b) | 12.13 | 2.75 | 1.05 | 0.80 | 82.12 | 1.20 | 2.2 |
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Share and Cite
Biondi, L.; Perry, M.; Vlachakis, C.; Wu, Z.; Hamilton, A.; McAlorum, J. Ambient Cured Fly Ash Geopolymer Coatings for Concrete. Materials 2019, 12, 923. https://doi.org/10.3390/ma12060923
Biondi L, Perry M, Vlachakis C, Wu Z, Hamilton A, McAlorum J. Ambient Cured Fly Ash Geopolymer Coatings for Concrete. Materials. 2019; 12(6):923. https://doi.org/10.3390/ma12060923
Chicago/Turabian StyleBiondi, L., M. Perry, C. Vlachakis, Z. Wu, A. Hamilton, and J. McAlorum. 2019. "Ambient Cured Fly Ash Geopolymer Coatings for Concrete" Materials 12, no. 6: 923. https://doi.org/10.3390/ma12060923